The present invention relates to new polycarbonate diols based on dimer diols, to the preparation of these polycarbonate diols and to their use as starting materials for high-value polyurethane plastics.
Aliphatic polycarbonate diols are by no means new. They are prepared from non-vicinal diols by reaction with diaryl carbonate (DE 1,915,908), dialkyl carbonate (DE 2,555,805), dioxolanones (DE 2,523,352), phosgene (DE 1,595,446), bis-chlorocarbonates (DE 857,948) or urea (P. Ball, H. Fuillmann and W. Heitz, Angew. Chem. 92 1980 no. 9, p 742, 743).
Of the diols described in the literature, only those based exclusively or predominantly on hexane-1,6-diol have previously gained any major industrial importance. For example, high-value polyurethane elastomers and coating agents are produced from hexanediol-polycarbonate according to known methods, which elastomers and coating agents are used for the production of articles with a long service life particularly on account of the outstanding resistance to hydrolytic influences. The resistance to hydrolysis of such polyurethanes is known to be orders of magnitude better than that of polyurethanes based on adipic acid polyester as diol component. Pure hexanediol-polycarbonates (having molecular weights of about 500 to 4,000) are waxes with a softening point of 45.degree.-55.degree. C. (depending on the MW). As a consequence of the crystallization tendency of the soft segment, the corresponding polyurethanes tend to harden at low temperatures and to lose their flexibility. In order to overcome this serious disadvantage, hexanediol-polycarbonates were prepared in which the softening point was reduced to a greater or lesser extent by incorporating interfering components, depending on their proportion. Since the relatively long-chain diols suitable for this purpose were not available industrially, adipic acid (DE 1,964,998), caprolactam (DE 1,770,245) or di-, tri- and tetraethylene glycol (DE 2,221,751) were used for said purpose. The result was a reduction in the resistance to hydrolysis of the polyurethanes due to the more readily hydrolyzing ester groups and the hydrophilic ether segments.
Polycarbonate polyurethanes (PU) are used increasingly in areas in which the articles produced from them are also exposed to hydrolytic influences aside from attack by microorganisms. This applies, e.g. to catheters, prostheses, jackets for cardiac pacemaker housings and cardiac pacemaker electrode leads.
Polyurethanes based on polyethers are known to be markedly more resistant to degradation by hydrolysis. The polymers of tetrahydrofuran exclusively suitable and also used for the above-mentioned fields of application with high quality requirements do, however, have some other disadvantages. In this case, too, the crystallinity leads to a hardening tendency on the part of the polyurethane at low temperatures, particularly if the desired range of properties of the polyurethanes requires the use of soft segments with an average MW of 2,000 and above. Only recently, has a particular disadvantage of polyurethanes based on polyethers been recognized. This disadvantage is that such polyurethanes tend to form cracks on the surface of molded articles when used in the body, contrary to the widely-held view, i.e. oxidative degradation of such products can no longer be excluded.
Polycarbonate polyurethanes are known from U.S. Pat. No. 5,229,431 from which molded articles which are said to be stable to oxidation are prepared. Moreover, polycarbonate polyurethanes are known from WO 92/04390 which are said to be suitable for the preparation of molded articles and disclosed as being biostable in in vitro and in vivo tests. A disadvantage of these polycarbonate polyurethanes, however, is that they are by no means as stable to hydrolysis as is required for long-term application. This is also confirmed by the in vitro hydrolysis and oxidation tests in the comparison examples of the present application.
Moreover, polyether polycarbonate diols are described in EP-B 0,292,772 which can be used for the preparation of polyurethane plastics. The polyurethane plastics prepared in this way need to be improved, however, with regard to their stability to hydrolysis and in particular to oxidation. As the comparison examples of the present application also show, ether groups, in particular, in the soft segment are susceptible to oxidative degradation. Polyether polycarbonate diols therefore have corresponding disadvantages.
Surprisingly, it was found that very suitable polyol components are obtained for the preparation of high-value polyurethane plastics which are superior to the polyurethane plastics containing conventional aliphatic polycarbonates in terms of stability to hydrolysis and oxidation, and which do not, moreover, have the above-mentioned disadvantages if dimer diols containing either 36 or 44 carbon atoms are reacted with suitable derivatives of carbonic acid to form relatively long-chain polycarbonate diols.